A conventional media access control system utilizing a token passing method in an optical LAN system has been described on pages 238 to 256 of "Proceedings of IEEE", Vol. 77, 1988. In this optical LAN system, a plurality of node apparatus are connected by optical fibers to provide a loop type optical network system. Each of the node apparatus comprises an access processing circuit for carrying out the processing of an access to a node apparatus, an optical to electric (0/E)converter for converting an optical signal received from an optical fiber transmission line to an electric signal to be supplied to the access processing circuit, and an electric to optical (E/0) converter for converting an electric signal supplied from the access processing circuit to an optical signal to be supplied to the optical fiber transmission line, and is assigned with an address to be discriminated from the other node apparatus. If it is assumed that the number of node apparatus is four, the first to fourth node apparatus are assigned with address signals "00", "01", "10" and "11" by using two bit signals.
In operation, when the first node apparatus receives a data transmission request to the third node apparatus, the access processing circuit of the first node apparatus captures a token packet which is converted to electric signals in the 0/E converter of the first node apparatus. In this access processing circuit, the data packet is processed to include the address signal "10" of the third node apparatus at a header portion thereof, and data to be transmitted from the first node apparatus to the third node apparatus at a data portion thereof. Thus, the token packet and the data packet are supplied from the access processing circuit of the first node apparatus via the E/0 converter thereof to the optical fiber transmission line, through which the token and data packets are transmitted to the third node apparatus. Thus, the data packet is transmitted from the first node apparatus to the third node apparatus. In this data packet transmission, the data packet is not pulled into the second node apparatus, because the address signal "10" at the header portion of the data packet does not coincide with the address signal "01" of the second node apparatus.
However, the conventional access control system in the optical LAN system has a disadvantage in that a token packet is converted between optical and electric signals and is subjected to the access processing even in a node apparatus having no transmission request, and a data packet is also converted between optical and electric signals and is subject to the access processing even in a node apparatus other than a node apparatus receiving data transmitted from another node apparatus. As a result, where there are a number of node apparatus having transmission requests, it takes a long time for a certain node apparatus to acquire a token packet, thereby lowering a through-put of the optical LAN system. In addition, there is a disadvantage in that jitter occurs in the access processing circuits of the node apparatus for processing a data packet electrically to deteriorate a bit error rate characteristic in accordance with an amount of the jitter proportional to the number of the node apparatus through which the data packet is transmitted. This results in the limitation on the number of node apparatus included in an optical LAN system. Further, there is a disadvantage in that a damage of either 0/E converter, access processing circuit or E/0 converter in any of node apparatus spreads to the whole network, because all the data packets are processed in the above components.